The delivery of natural gas from the site of extraction to the end consumer presents a significant logistical challenge. Pipelines can be used to transport natural gas over short distances (typically less than 2000 km in offshore environments and less than 3800 km in onshore environments), but they are not an economical means of transport when larger distances are involved. Furthermore, it is not practical to build pipelines in certain environments, such as, for example, across large expanses of water.
It is more economical to transport liquefied natural gas (LNG) over very large distances and in situations where delivery to a number of different destinations is required. The first stage in the liquefied natural gas delivery chain involves the production of the natural gas. The natural gas is then transferred to a LNG production plant where it is liquefied prior to transportation (typically by shipping). The liquid natural gas is then re-vaporised at the destination and distributed to the end consumers by pipeline delivery.
The liquefaction of natural gas is achieved by exposing a natural gas feed stream to one or more refrigeration cycles. These refrigeration cycles can be extremely energy intensive, primarily due to the amount of shaft power input required to run the refrigerant compressors.
A number of refrigeration processes for liquefying natural gas are known in the art. One well established approach involves the cooling and condensing a natural gas feed gas stream in one or more heat exchangers against multiple refrigerant streams provided by re-circulating refrigeration systems. Cooling of the natural gas feed is accomplished by various cooling process cycles, such as the well known cascade cycle in which refrigeration is provided by three different refrigerant loops. One such cascade cycle uses methane, ethylene and propane cycles in sequence to produce refrigeration at three different temperature levels. Another well-known refrigeration cycle uses a propane pre-cooled, mixed refrigerant cycle in which a multi-component refrigerant mixture generates refrigeration over a selected temperature range. The mixed refrigerant can contain hydrocarbons such as methane, ethane, propane and other light hydrocarbons, and also may contain nitrogen. Versions of this refrigeration system are used in many operating LNG plants around the world.
One of the simplest refrigeration systems comprises a single mixed refrigerant cycle (e.g. the Black & Veatch PRICO process). One problem with such processes is that they exhibit lower thermodynamic efficiency relative to more complex processes (e.g. the propane-cooled mixed refrigerant cycle by Air products, or the double mixed refrigerant cycle by Shell). Furthermore, the thermodynamic performance and efficiency of a single mixed refrigerant cycle can only be varied by adjusting a small number of operating variables, such as the refrigerant composition, the condensation and evaporation temperature and the pressure level. The more complex multi-cycle processes are able to offer improved cycle efficiency by providing more operating variables, including, for example, varying the composition and temperature of multiple refrigerant streams, which can significantly affect the exergy loss in heat exchangers. By properly adjusting these additional operating variables, the thermodynamic efficiency can be significantly improved in these more complicated refrigeration processes when compared with a single mixed refrigerant cycle. However, multi-stage or cascade refrigeration processes usually require much more complicated equipment configurations, and this results in significant plant and equipment costs.
Consequently, there is a balance to be struck between providing a refrigeration process that is simple in design and construction, and thereby saves on plant and equipment costs, and providing a process which also possesses sufficient operating variables to enable satisfactory and/or improved operating efficiency.
The present invention seeks to provide refrigeration processes that address one or more of the aforementioned drawbacks by providing a single cycle, mixed refrigeration process which comprises additional operating variables to enable the provision of improved operating efficiency.